An investigation of the transition from polysilicon emitter to SIS emitter behavior

The electrical effects of scaling the emitter-base junction depth of polysilicon emitter bipolar transistors are investigated. A range of devices with varying amounts of emitter dopant penetration into the single-crystal silicon has been fabricated by varying the emitter drive-in conditions. Detailed electrical measurements of the base and collector currents as a function of base-emitter voltage have been made and show that the depth of the single-crystal emitter region is important in determining the electrical characteristics. The ideality of the base current severely degrades as the single-crystal emitter depth decreases, and at forward voltage around 0.9 V a kink is often observed. The collector characteristics are ideal at low forward voltages, but at high voltages the collector current saturates, with the saturation occurring at lower currents as the single-crystal emitter depth decreases. A numerical model for shallow polysilicon emitters (termed SIS emitters) has been developed, and is used to explain these nonideal device characteristics in terms of recombination, and direct and indirect tunneling at the polysilicon-silicon interface. Good quantitative agreement between theory and experiment is obtained and values for the model parameters can be obtained by fitting to the experimental results.>